This invention relates to testers using capacitive cells to measure permittivity of materials for determining the content of substances having a distinctive dielectric quality. Testers of this type include those having capacitors shaped like containers or cells into which are placed grains or other particulate materials for measuring their moisture content.
Typically, a type of tester for determining the amount of moisture in a particulate material includes a capacitive cell having an inner cylindrical, insulated electrode of quite small diameter surrounded by an outer electrode of substantially greater diameter. The testers may be portable and adapted for testing materials such as grain in discrete, accurately measured samples poured in the space between the inner and the outer electrodes, or the tester may be permanently installed in a process or handling facility for continually testing a portion of flowing material as it is diverted over a short distance from the main flow to flow downwardly between the electrodes. In either type of tester, the capacitance or impedance of the cell is measured electronically for determining the permittivity of the material, and the reading for permittivity is converted to a reading of moisture either directly on a scale of an output meter or by reference to a conversional table.
Usually, the capacitive cell of the tester is included in a voltage divider, and voltage of repetitive waveform at a single frequency is applied across the voltage divider. The amount of moisture in a material such as grain is determined by measuring the voltage developed across the cell. The readings obtained on a tester of this type are usually valid when the moisture is quite evenly distributed within the kernels of the grain. A different calibration may be required if the moisture is contained in the grain in other ways; for example, the moisture may be contained within the grain not only in a close molecular condition but also it may be contained on the surface of the grain or in capillaries within the kernels either in liquid or in frozen form. Tests show that the permittivity of grain measured in a usual manner at a single frequency appears to be greater for a certain total amount of moisture when a greater portion of the moisture is contained on its surface.
The output readings of testers for measuring permittivity of materials placed in capacitive test cells are affected in various amounts by conditions of the materials such as density and temperature and of composition such as the amount of protein in grain. For temperature correction, temperature sensing devices in contact with the material being tested can control circuits for compensating the output readings of the testers. The amount of materials in the test cells are carefully controlled to maintain density nearly constant. Although the amounts of protein have usually been disregarded in grain moisture testers, changes in output readings with changes in protein in capacitive testers are being investigated not only for increasing the reliability of measurements of moisture but also as possibly providing a quick, reliable means of measuring grain for its amount of protein.